Low conductivity and sintering-resistant thermal barrier coatings

ABSTRACT

A thermal barrier coating composition is provided. The composition has a base oxide, a primary stabilizer, and at least two additional cationic oxide dopants. Preferably, a pair of group A and group B defect cluster-promoting oxides is used in conjunction with the base and primary stabilizer oxides. The new thermal barrier coating is found to have significantly lower thermal conductivity and better sintering resistance. In preferred embodiments, the base oxide is selected from zirconia and hafnia. The group A and group B cluster-promoting oxide dopants preferably are selected such that the group A dopant has a smaller cationic radius than the primary stabilizer oxide, and so that the primary stabilizer oxide has a small cationic radius than that of the group B dopant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/904,084 filed Jul. 12, 2001, now U.S. Pat. No. 6,812,176which claims the benefit of U.S. provisional patent application Ser. No.60/263,257 filed Jan. 22, 2001.

STATEMENT OF GOVERNMENT SPONSORED RESEARCH

This invention was made with government support under Contract No.NCC3-617 awarded by NASA. The government has certain rights in thisinvention.

FIELD OF THE INVENTION

The invention relates to a low conductivity thermal barrier coating, inparticular to such a coating containing zirconia and/or hafnia, and acombination of rare earth oxides and/or certain other oxides.

BACKGROUND OF THE INVENTION

Thermal barrier coatings are thin ceramic layers that are used toinsulate air-cooled metallic components from high temperature gases, forexample in gas turbine or other heat engines. Such coatings are usefulin protecting and extending the service life of metallic componentsexposed to high temperatures, such as jet engine turbine blades andcombustor components. Thermal barrier coatings composed ofyttria-stabilized zirconia are known, wherein the yttria typically makesup seven to nine weight percent (or four to five molar percent) of thetotal composition. These coatings are typically applied using plasmaspraying or physical vapor deposition techniques in which melted ceramicparticles or vaporized ceramic clouds are deposited onto the surface ofthe component that is to be protected. Thermal barrier coatings aresomewhat porous with overall porosities generally in the range of 5 to20%. This porosity serves to reduce the coating's effective thermalconductivity below the intrinsic conductivity of the dense ceramic, aswell as to improve the coating's strain tolerance. However, the coatingconductivity will increase as the porosity decreases in high temperatureservice due to ceramic sintering.

In a jet engine, higher operating temperatures generally lead to greaterefficiency. However, higher temperatures also cause more problems suchas higher stresses, increased materials phase instability and thermaloxidation, leading to premature failure of components. A ceramic coatingwith lower thermal conductivity and improved high temperature stabilitywould allow higher operating temperatures while preserving operatinglife of the coated component. Accordingly there is a need for thermalbarrier coatings with a lower conductivity and better sinteringresistance than prior art coatings. Such a coating ideally would retainlow conductivity after many hours of high temperature service. A lasertest, recently developed by the current inventors has allowedsimultaneous testing of durability conductivity, and conductivityincrease due to sintering under turbine-level high heat flux conditions.See, e.g., Dongming Zhu and Robert A. Miller, “Thermal Conductivity andElastic Modulus Evolution of Thermal Barrier Coatings under High HeatFlux Conditions,” Journal of Thermal Spray Technology Vol. 9(2) Jun.2000 pp. 175–180, and “Thermophysical and Thermomechanical Properties ofThermal Barrier Coating systems,” Ceramic Engineering and ScienceProceedings, Vol. 21, 2000 pp. 623–633, both of which are incorporatedherein by reference. The thermal barrier coating advances described inthis application have had the benefit of this new test approach.

SUMMARY OF THE INVENTION

A thermal barrier coating composition is provided, having 46–97 molarpercent base oxide, 2–25 molar percent primary stabilizer, 0.5–25 molarpercent group A dopant, and 0.5–25 molar percent group B dopant. Thebase oxide is selected from the group consisting of ZrO₂, HfO₂ andcombinations thereof; the primary stabilizer is selected from the groupconsisting of Y₂O₃, Dy₂O₃, Er₂O₃ and combinations thereof; the group Bdopant is selected from the group consisting of Nd₂O₃, Sm₂O₃, Gd₂O₃,Eu₂O₃ and combinations thereof, and the group A dopant is selected fromthe group consisting of rare earth oxides, alkaline earth metal oxides,transition metal oxides and combinations thereof, but excluding thosespecies contained in the base oxide, group B dopant and primarystabilizer groups. The ratio of the molar percentages of group A dopantto group B dopant in the composition is between about 1:10 and about10:1.

A further thermal barrier coating composition is provided, having aceramic alloy solid solution having a base oxide lattice structure orstructures where the base oxide is present in the solid solution in anamount of 46–97 molar percent. The solid solution further includes 2–25molar percent primary stabilizer, 0.5–25 molar percent group A dopant,and 0.5–25 molar percent group B dopant. The base oxide is selected fromthe group consisting of ZrO₂, HfO₂ and combinations thereof, and theprimary stabilizer is selected from the group consisting of Y₂O₃, Dy₂O₃,Er₂O₃ and combinations thereof. Each of the group A dopant and the groupB dopant is selected from the group consisting of rare earth oxides,alkaline earth metal oxides, transition metal oxides and combinationsthereof, but excluding those species contained in the base oxide andprimary stabilizer groups. The group A dopant is selected such that theionic radius of the group A dopant cation is smaller than the ionicradius of the primary stabilizer cation in the solid solution. The groupB dopant is selected such that the ionic radius of the group B dopantcation is larger than the ionic radius of the primary stabilizer cationin the solid solution. The ratio of the molar percentages of group Adopant to group B dopant in the solid solution is between about 1:10 andabout 10:1.

A turbine blade structure also is provided. The turbine blade structurehas a metallic turbine blade substrate and a ceramic thermal barriercoating thereover. The ceramic thermal barrier coating has a ceramicalloy solid solution having 46–97 molar percent base oxide, 2–25 molarpercent primary stabilizer, 0.5–25 molar percent group A dopant, and0.5–25 molar percent group B dopant. The base oxide is selected from thegroup consisting of ZrO₂, HfO₂ and combinations thereof, the primarystabilizer is selected from the group consisting of Y₂O₃, Dy₂O₃, Er₂O₃and combinations thereof; and each of the group A dopant and the group Bdopant is selected from the group consisting of rare earth oxides,alkaline earth metal oxides, transition metal oxides and combinationsthereof, but excluding those species contained in the base oxide andprimary stabilizer groups. The group A dopant is selected such that theionic radius of the group A dopant cation is smaller than the ionicradius of the primary stabilizer cation in the solid solution, and thegroup B dopant is selected such that the ionic radius of the group Bdopant cation is larger than the ionic radius of the primary stabilizercation in the solid solution. The ratio of the molar percentages ofgroup A dopant to group B dopant in the solid solution is between about1:10 and about 10:1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectioned side view of a metallic substrate having a ceramicthermal barrier coating according to the invention applied thereto. Ametallic bond coat layer, which is optional but highly preferred, isprovided intermediate the thermal barrier coating and the substrate. Thesubstrate can be, e.g., a turbine blade for a jet engine, a combustor,or any other component that is subjected to high temperature service,particularly high temperature thermal cycling as in a jet engine.

FIG. 2 is a sectioned side view as in FIG. 1, incorporating anadditional ceramic layer intermediate the metallic bond coat layer andthe thermal barrier coating layer according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

As used herein, all percentages are given as molar percents unlessotherwise indicated. As used herein, when a preferred range such as 5–25(or 5 to 25) is given, this means preferably at least 5 and, separatelyand independently, preferably not more than 25.

Also as used herein, the intrinsic or material thermal conductivity of amaterial refers to the material's thermal conductivity measured as aphysical property of a nonporous, dense solid sample of the material,whereas the effective or superficial thermal conductivity of a materialrefers to the material's effective thermal conductivity measured for aporous sample of the material. Generally, a porous material's effectiveor superficial thermal conductivity will be lower than the material'sintrinsic thermal conductivity because the porous network of thematerial limits or reduces the pathways thermal energy can take throughthe material.

Also as used herein, the notation x-y-z, where x, y and z each is adecimal or whole number such as 4 or 1.33 or 1.67, refers to acomposition for a ceramic alloy solid solution or thermal barriercoating having x mol. % primary stabilizer, y mol. % group A dopant andz mol. % group B dopant, with the balance being base oxide as thosecomponents are described herein.

The present invention provides a substantial improvement to ceramicthermal barrier coatings that have been used to coat high temperatureservice components, for example in jet engines. Such components include,but are not limited to, turbine blades as well as combustor components.As explained above, these components conventionally have been coatedwith a ceramic coating of yttria-stabilized zirconia, typically about3.9–4.55 mol. % Y₂O₃ (about 7–8 wt. %), balance ZrO₂ and impurities. Forplasma sprayed coatings, as initially laid down the coatings providesuitably low effective thermal conductivity, k˜1.0 W/m-K. However, theeffective thermal conductivity of these coatings tends to increaserather markedly after several hours of high temperature service, andvalues of 1.15, 1.19 and 1.5, W/m-K have been observed experimentallyafter 30 hours of service for such coatings having 0.25 mm thickness attemperatures of 990, 1100 and 1320, ° C., respectively. This observedincrease in thermal conductivity (from 15% to as high as 50% after only30 hours) dramatically shortens the useful life of the coated partbecause the part is exposed to far greater thermal stress as a result ofmore thermal energy being conducted through the coating. The thermalbarrier coating becomes less effective at performing its thermalshielding function because its capacity to conduct thermal energy to thecoated part increases with time under high temperature conditions.

The observed increase in effective thermal conductivity in theconventional plasma sprayed ceramic coatings is due to sintering of theceramic material at the elevated service temperatures, which essentiallycloses up or “fills in” the porous network of the ceramic layer. As aresult of the reduced porosity, the effective thermal conductivity ofthe layer approaches the dense ceramic's intrinsic thermal conductivity(about 2.5 W/m-K for 4.55 mol. % yttria-stabilized zirconia) as thecoating continues to become more continuously solid and less porous. Inaddition, microcracks (on the order of 1 to 10 microns or less) presentin the native, as-applied ceramic coatings, particularly those depositedvia plasma spray, are believed to be an efficient barrier to conductiveheat transfer through the coating, contributing to the overall loweffective conductivities observed for these coatings. The finer of thesemicrocracks, (about 1 micron width or less) can be healed or filled inas a result of sintering, further elevating the effective thermalconductivity toward the dense ceramic's intrinsic conductivity. Also, asthe coating becomes more dense due to sintering it exhibits acorresponding increase in its elastic modulus, thus increasing theelastic strain energy in the coating and causing reduced durabilityespecially upon thermal cycling.

The present invention is based on the discovery that the incorporationof certain dopant oxides in various proportions into the ceramic coatingmatrix can stabilize the porous ceramic coating against sintering via adefect clustering effect. In addition, it has been found that certaincombinations of dopant oxides also impart a greater stability againstthermal cycling to the ceramic coating matrix. The result of the aboveeffects is that a high-temperature service component that is coated witha ceramic thermal banier according to the present invention will have asubstantially improved useful service life, can be operated at highertemperature, and will be more resistant to thermal cycling resultingfrom, e.g., an aircraft's flight schedule. It also has been found thatcertain combinations of dopant oxides may impart enhanced erosionresistance to the thermal barrier coatings. The coatings according tothe invention, and particularly the dopant oxides referred to above, aredescribed in detail below.

In its broadest aspect, a thermal barrier coating according to theinvention is comprised of a ceramic alloy solid solution having at leastthe following four components: 1) a base oxide, 2) a primary stabilizer,3) a group A dopant, and 4) a group B dopant.

In a first preferred embodiment, the thermal barrier coating accordingto the present invention has the following preferred formulation ortable of components listed in table 1. In table 1, any preferred or lesspreferred molar percent or molar percent range for any component can becombined with any preferred or less preferred molar percent or molarpercent range for any of the other components; it is not necessary thatall or any of the molar percents or molar percent ranges come from thesame column.

TABLE 1 TABLE OF COMPONENTS MOLAR PERCENTS Less Preferred Less PreferredPreferred Base Oxide 60–93 82–91 86 46–97 73–92 84–89 Primary  4–17 6–12 8–9 Stabilizer  3–20  4–14  7–10  2–25 Group A 0.9–8   1.75–4.5 2.5 Dopant 0.8–9   1.5–5   0.7–10  1.25–6   0.5–12  1–7  0–25 Group B0.9–8   1.75–4.5  2.5 Dopant 0.8–9   1.5–5   0.7–10  1.25–6   0.5–12 1–7  0–25

The base oxide is the primary component in the solid solution, meaningthat the base oxide is present therein in an amount preferably greaterthan 50 mol. %, more preferably 60 mol. %, more preferably 70 or 80,mol. %. In any event, the base oxide should represent the highest mol. %concentration in the ceramic alloy solid solution relative to any othersingle component therein. The base oxide represents the balance of theceramic alloy solid solutions described herein, excepting impurities,after accounting for the molar percent concentrations of the primarystabilizer, group A dopant and group B dopant, which collectively makeup the total dopant oxides. For example, a ceramic alloy solid solutionhaving 4 mol. % primary stabilizer, 3 mol. % group A dopant and 3 mol. %group B dopant, has 90 mol. % base oxide and impurities (100 mol. %−4mol. %−3 mol. %−3 mol. %=90 mol. %). As used herein, whenever a baseoxide is said to represent the “balance” of a ceramic alloy solidsolution or ceramic thermal barrier coating, it is to be understood thatthis is not intended necessarily to exclude the presence of anyimpurities.

The base oxide most preferably is selected from either zirconia (ZrO₂),hafnia (HfO₂) or a combination of these. Zirconia may be preferredbecause of its relatively low cost, wide availability and the fact thatcoating manufacturers already are broadly familiar with zirconia-basedceramic thermal barrier coatings, including techniques for theirapplication to desired substrates. However, the applicants have observedvery good sintering resistance and low thermal conductivity forhafnia-based coatings as well when doped according to the invention.

The primary stabilizer is a species effective to fully or partially“stabilize” the cubic phase of the base oxide. This avoids therelatively large volume change associated with phase transformationduring heating or cooling the pure base oxide, e.g. zirconia, as ittransforms from the low temperature monoclinic phase to a highertemperature tetragonal phase. A fully stabilized base oxide will becubic across a broad temperature range. A partially stabilized baseoxide may be partly cubic or it may contain a certain quenched-in phaseknown as t′ (the tetragonal t′ phase described elsewhere herein), forexample t′-zirconia. Oxide stabilizers generally cause a decrease in theintrinsic thermal conductivity of the base oxide ceramic (zirconia orhafnia) as a result of increased vacancies and atomic weight associatedwith the stabilizing oxide additions. Such stabilizers, as well as theireffect of lowering intrinsic thermal conductivity, are well known in theart. For example, it is known that the addition of yttria (Y₂O₃) inminor amounts to zirconia (ZrO₂) causes the material thermalconductivity of the primarily zirconia matrix to be reduced. This hasbeen the basis for conventional ceramic thermal barrier coatings, whichutilize 4.55 mol % (˜8 wt. %) yttria-stabilized zirconia, commonlyreferred to in the art as “8YSZ”. Conventionally, yttria (Y₂O₃) is themost common primary stabilizer species, and ceramic coatings made from4.55 mol. % yttria-stabilized zirconia (8YSZ) are well known in the art.Alternatively, the primary stabilizer can be any other species presentin an amount effective to stabilize, and therefore to reduce theintrinsic thermal conductivity of, the solid solution relative to thebase oxide alone, including dysprosia (Dy₂O₃), less preferably erbiumoxide (Er₂O₃), or combinations of these and/or yttria.

The group A and B dopants preferably are provided in smaller amountscompared to the base oxide, and are responsible for providing the defectclustering effect mentioned above, and more fully described below. Thegroup A and B dopants both are cationic oxides, preferably both rareearth oxides. The group A dopant is selected such that the ionic radiusof the group A dopant cation will be smaller than the ionic radius ofthe primary stabilizer cation in the solid solution. Conversely, thegroup B dopant is selected such that the ionic radius of the group Bdopant cation will be larger than the ionic radius of the primarystabilizer cation in the solid solution. In other words, the group Adopant cationic radius is smaller than that of the primary stabilizer,whose cationic radius in turn is smaller than that of the group Bdopant.

The group A dopant is preferably scandia oxide (Sc₂O₃) or ytterbiumoxide (Yb₂O₃), less preferably combinations of these, less preferablynickel (II) oxide (NiO), chromium (III) oxide (Cr₂O₃), cobalt (II) oxideCoO, iron (III) oxide (Fe₂O₃), magnesium (II) oxide (MgO), lesspreferably titanium (IV) oxide (TiO₂), ruthenium (IV) oxide (RuO₂),tantalum oxide (Ta₂O₅) or combinations of any of the foregoing, lesspreferably any other rare earth oxide (excluding erbium oxide (Er₂O₃)),alkaline earth metal oxide, transition metal oxide, or combinations ofany of the foregoing, so long as the cationic radius of the group Adopant is smaller than the cationic radius of the primary stabilizerthat is selected. Among scandia and ytterbia, ytterbia may be morepreferred due to its lower cost.

The group B dopant is preferably neodymium oxide (Nd₂O₃) or gadoliniumoxide (Gd₂O₃), less preferably samarium oxide (Sm₂O₃), less preferablyeuropium oxide (Eu₂O₃), or combinations of any of these.

Other additives known in the art may also be added in conventionalamounts. Because of the difficulty in separating rare earth metals fromeach other, the listed percentages indicate that the given component(base oxide, primary stabilizer dopant, group A dopant, group B dopant)consists essentially of the given metal oxide compound, with allowancefor impurities normally tolerated from commercially available sources.The components of the composition are combined according to techniquesknown or conventional in the art to form the desired ceramic thermalbarrier coating composition.

The group A and group B dopants are preferably present in substantiallyequal molar percentages in the solid solution for the thermal barriercomposition. Less preferably the group A and group B dopants are presentin a molar percent ratio between 1.1:1 to 1:1.1, less preferably 1.5:1to 1:1.5, less preferably 1:2 and 2:1, less preferably 1:4 to 4:1, lesspreferably 1:8 to 8:1, less preferably 1:10 to 10:1. It has beenexperimentally observed that the incorporation of the group A and groupB dopants together in the above proportions into a ceramic alloy solidsolution matrix having as its primary component zirconia and/or hafniaas the base oxide, in addition to a relatively minor amount of yttria,results in the solid solution matrix exhibiting a substantially reducedintrinsic thermal conductivity, as well as both a reduced effective orsuperficial thermal conductivity (which is lower than the intrinsicthermal conductivity), and a reduce tendency for the effective thermalconductivity to rise after hours of high-temperature service.

Without wishing to be bound by any particular theory, it is believedthat this highly surprising and unexpected result is achieved for atleast one of the following reasons.

As discussed above the present invention involves the addition of bothgroup A and group B dopant oxides to the zirconia (hafnia)-yttria orother zirconia (hafnia) based systems. The group A and group B dopantsare believed to serve three functions.

First, the group A and B dopants preferably are present in substantiallyequimolar ratios so as to provide essentially paired oxide complexes inthe base oxide (zirconia) lattice. The group A and group B dopant oxidesare generally highly stable, possessing respectively smaller and largercationic radii than that of the primary stabilizer dopant, mostpreferably yttria. In the base oxide solid solution lattice, thesepaired oxides (groups A and B) cooperate to provide thermodynamicallystable, highly distorted, defective lattice structures based on localionic segregation (defect clustering) with controlled ranges of defectcluster sizes, resulting in a highly distorted lattice in the ceramicalloy solid solution. These complex structures effectively attenuate andscatter lattice phonon waves as well as radiative photon waves at a widespectrum of frequencies, thus significantly reducing the oxide latticeintrinsic and radiation thermal conductivities.

Second, the resulting defect clusters are substantially immobile in thesolid solution's highly distorted lattice structure. These essentiallyimmobile defect clusters, themselves or in combination with otherinduced nanoscale ordered phases, effectively reduce the mobile defectconcentration in the base oxide lattice and suppress atomic mobility andmass transport, thus significantly improving the oxide sintering-creepresistance.

Third, it has been observed that ceramic thermal barrier coatings madeas described herein exhibit substantially improved mechanicalproperties, including improved fracture toughness as well as durability,believed due to the formation of complex nanoscale defect clusters.

The one or more pairs of additional dopants (group A and B dopantoxides) incorporated in the base oxide matrix (preferably ZrO₂ or HfO₂based oxides) preferably have cationic valences that are either lessthan or greater than that of the base oxide cation, less preferablyequal to that of the base oxide cation. For example, in the preferredembodiment where ZrO₂ is the base oxide, the zirconium cation has a +4valence, Zr⁴⁺, meaning the dopant oxides (group A and B dopants)preferably are selected having cationic valences of +1, +2 or +3, whichare less than that of zirconium in ZrO₂, or valences of +5 or larger,which is greater than that of zirconium in ZrO₂. It is less preferred inthe case where zirconia is the base oxide to have a group A or B dopantoxide whose cationic valence is equal to +4. In any case, however, it isimportant that the dopant oxides have a substantially different cationicradius than that of the base oxide (Zr⁴⁺ in the preferred embodiment) inorder to achieve the desired defect clustering effect. By selecting thegroup A and B dopants according to the valence considerations describedin this paragraph, a high concentration of highly associated defects isproduced in the solid solution matrix which is stabilized based onelectroneutrality conditions. Strong dopant-oxygen vacancy anddopant-dopant interactions are believed to exist which promote theformation of extended, immobile defect clusters.

It was mentioned above that the group A and B dopant oxides are selectedin part based on their high stability. Stability of these oxides isgoverned by their free energy of formation (ΔG_(f)), where a more(higher magnitude) negative value indicates greater stability as is wellknown in the art. The one or more pairs of additional dopants, (group Aand B dopant oxides are one such pair), incorporated in the base oxidematrix (ZrO₂ and/or HfO₂ based systems) preferably are selected so as topossess high (large magnitude negative value) free energies offormation. High free energy of formation of a dopant oxide representshigh stability of the oxide in an oxygen-containing atmosphere, which isgreatly desirable. The addition of dopant oxides with mixed polarizationand ionic sizes may further introduce lattice defects and distortion forreduced thermal conductivity and improved sintering-creep resistance.

High dopant atomic weight is beneficial for improving latticescattering. High dopant solubility is beneficial for increasing dopantconcentrations and thus defect concentrations. A mixture of high- andlow-solubility dopants and of high- and low-atomic weight will promotecomplex structures and thus improved lattice scattering.

The compositions according to the invention preferably are used toprovide a ceramic thermal barrier coating on metal components subject tohigh service temperatures, such as turbine blades in jet engines. Thecoatings may be applied to the substrate using methods known in the art,for example via a plasma spray technique or physical vapor deposition.Most preferably, a metallic bond coat layer made from or substantiallycomprising an oxidation-resistant metal or metal alloy is depositedfirst onto the substrate surface, and the ceramic thermal barriercoating deposited over the bond coat layer. Metallic bond coat layersare known in the art and typically are applied via a conventional vapordeposition technique to a thickness of about 3–7, more typically about5, mils. Suitable bond coat materials are MCrAlX alloys as known in theart, where M≡is a metal selected from nickel, cobalt, iron, andcombinations thereof; Cr≡chromium; Al≡aluminum; and X≡an optional activemetallic element, most commonly yttrium. Pt—Al (platinum-aluminum)alloys also are common for the bond coat layer. Broadly, the bond coatlayer typically is an aluminum alloy where aluminum provides theoxidation resistance to the layer, and the other alloying elements arepresent in amounts sufficient to improve (reduce) the brittleness andraise the melting temperature of the bond coat above the servicetemperature for the coated substrate. The bond coat layer is highlypreferred in order to prevent oxidation of the underlying metallicsubstrate which often is not all that oxidation resistant. For example,turbine blades commonly are manufactured of a nickel superalloy thatwill be readily oxidized from atmospheric oxygen at operatingtemperatures over 2000° F. The bond coat is preferred because a thermalbarrier coating alone may not sufficiently prevent oxidation of theunderlying substrate because such coatings, often comprising primarilyporous zirconia in the coating layer, can be oxygen permeable and/oralso relatively effective oxygen conductors at elevated temperatureseven when the coating layer is dense. Once the bond coat layer has beendeposited onto the substrate, the ceramic thermal barrier coatingcomposition is applied over the bond coat layer via conventionaltechniques to provide the ceramic thermal barrier coating according tothe invention.

Most preferably, the composition is applied such that the resultingcoating exhibits primarily a crystalline phase that is either cubic,tetragonal t′ or primarily a combination of these. The cubic phase of acrystalline ceramic such as zirconia or hafnia, as well as for otheroxides encompassed by the present description, is very well known in theart and does not require further explanation here. The tetragonal t′phase referred to herein also is known to those skilled in this art, butis somewhat less common and bears some explanation for the convenienceof the reader. The tetragonal t′ phase of a ceramic crystal is anon-equilibrium state of the crystal exhibiting a tetragonal crystallinestructure which generally is not apparent from the conventional phasediagram for the crystal, e.g. phase diagrams for conventional ZrO₂—Y₂O₃systems do not show this phase. It results when the base materialbecomes supersaturated with the dopant, conventionally Y₂O₃, in the caseof the invention the sum of all the dopants. It is a non-equilibrium ormetastable state and is converted to the equilibrium cubic phase asadditional dopant(s) is/are added. Though it is a non-equilibrium state,the tetragonal t′ phase of zirconia or hafnia generally persists up totemperatures of about 1200° C. (2192° F.) at which point it will begradually converted to the more stable cubic phase. The tetragonal t′phase inevitably also includes substantial micro-scale regions of otherphases scattered throughout the bulk phase, which result in improveddefect scattering and the resultant improved durability described below.

It is to,be understood that nothing in the foregoing paragraph orelsewhere in this application is intended to imply that only the cubicand/or tetragonal t′ phases will or are to be present in the solidsolutions for the ceramic coatings of the present invention. The solidsolutions inevitably will have regions or pockets of various crystallinephases, including cubic, tetragonal, tetragonal t′, monoclinic, etc.,depending on the conditions under which a particular coating wasdeposited and its service environment. In addition, a coating may havetwo or more discrete regions having the same crystalline phase, butwhich are not continuous—i.e. the lattice structures of two adjacentphases may both be cubic, but the two phases nevertheless arediscontinuous and meet at a discrete interface as known in the art.

It has been found in applications using the ceramic thermal barriercoatings according to the invention that cubic phase coatings generallyhave lower intrinsic thermal conductivity, better phase stability, andalso higher concentrations of defect clusters as described above whichresult in greater sintering stability across a very wide temperaturerange. On the other hand, tetragonal t′ phase coatings have been foundto be more durable and resistant to erosion from physical impact events(i.e. they are tougher coatings), and also have exhibited greaterresistance to thermal cycling compared to counterpart cubic phasecoatings. It will be understood by persons of ordinary skill in the artthat the choice of whether to provide a substantially cubic or asubstantially tetragonal t′ phase coating will greatly depend on theparticular application, such as the temperatures, number and frequencyof actual or potential physical impact events and/or degree of thermalcycling experienced by the substrate to be coated. Of course, in certainapplications it also may be desirable to provide a coating having somemeasure of both cubic and tetragonal t′ phases in order to provide thebenefits of each of these phases.

The zirconia-based ceramic thermal barrier coatings having asubstantially cubic phase described above exhibit substantially reducedintrinsic thermal conductivity as already discussed, and also haverelatively high thermal expansion coefficients. The combination of thesetwo properties, along with a lower degree of toughness compared to thetetragonal t′ phase counterpalts, can increase the potential for thermalshock damage and premature delaminations, especially near edges or sharpbends in coated components (e.g. the edge of a turbine blade) where highstress concentrations and non-uniformity of the temperaturedistributions are to be expected. To counteract this potentiallynegative effect, it is preferable first to lay down a base ceramic layerprior to depositing the thermal barrier coating according to theinvention. The base ceramic layer advantageously can be a conventional7YSZ or 8YSZ (7 or 8 weight percent respectively, yttria-stabilizedzirconia) layer, and preferably has a thickness in the range of 0.5–2mils (about 0.5–1 mil for a PVD applied base ceramic layer). Such a 7YSZor 8YSZ coating has a somewhat lower thermal expansion coefficient, andalso a relatively higher thermal conductivity compared to thesuperjacent cubic zirconia-based thermal barrier coating according tothe invention, and its properties generally are intermediate theunderlying metallic bond coat layer (or substrate) and the thermalbarrier coating. Such a two-layer ceramic coating system (base ceramiclayer of conventional 7- or 8YSZ ceramic and superjacent thermal barrierlayer according to the invention) has demonstrated significant increaseduseful life and resistance to thermal cycling, while maintaining optimumlow thermal conductivity performance and temperature capability, interms of both intrinsic conductivity and sintering resistance for thesuperjacent thermal barrier coating layer. Additional layer gradingbetween these two ceramic layers in terms of compositions and propertiesalso can be incorporated in order to improve the two coatings' stabilityand compatibility. Alternatively, the base ceramic layer can be atetragonal t′ phase low conductivity thermal barrier coating layer,and/or can have an as-applied thickness in the range of 10 to 50 μm.This base ceramic layer is preferred for zirconia-based thermal barriercoatings according to the invention, and not necessarily forhafnia-based coatings, though it is believed the base ceramic layerwould not be detrimental to a hafnia-based coating.

Generally, when zirconia is used as the base oxide, total dopantconcentrations (sum of primary stabilizer, group A and group B dopants)greater than about 6 mol. % result in the ceramic thermal barriercoating being substantially in the cubic phase. Total dopantconcentrations in the range of 4–6 mol. % result in a mixed phasecoating having substantial regions of both cubic and tetragonal t′phases, and total dopant concentrations less than about 4 mol. % resultin a substantially tetragonal t′ crystalline phase for the coating. Whenhafnia is used as the base oxide, it has been found that the cubic totetragonal t′ phase transition occurs in the range of about 10–15 mol. %total dopant concentration, where 15 mol. % results in a substantiallycubic phase coating and below 10 mol. % results in a substantiallytetragonal t′ phase.

The inventors herein have identified a number of compositions whichprovide highly desirable ceramic thermal barrier coatings according tothe invention, for the case where either zirconia or hafnia is selectedas the base oxide. Zirconia is relatively inexpensive and less heavycompared to hafnia and so may be preferred in a wide range ofapplications. It is generally suitable in applications up to about 2800°F. (1540° C.). However, where it is necessary to provide a coating for asubstrate that will experience higher temperatures, the higher cost ofhafnia may be justified, which generally is suitable in applicationsabove 2800° F.

When zirconia is selected as the base oxide, it has been found,surprisingly and unexpectedly, that total dopant concentrations in therange of about 9–13 mol. %, more preferably about 10–12 mol. %, resultin a dramatic reduction in intrinsic thermal conductivity for theceramic coating. This, coupled with enhanced sintering resistance due tothe defect clustering effect resulting from the paired oxides (group Aand B dopants), results in a superior ceramic thermal barrier coatinghaving 1) lower intrinsic thermal conductivity, 2) lower effectivethermal conductivity in the porous coating, and 3) a far greater servicelife because the low effective thermal conductivity is sustained for alonger period of time due to the enhanced sintering resistance of thematerial. For zirconia-based systems, the following compositions intable 2 have been identified as producing highly desirable coatings. Intable 2, all values are molar percent.

TABLE 2 Primary Group Group Base Oxide Stabilizer A Dopant B Dopant(zirconia) 4 3 3 Balance 4 4 4 Balance 6 3 3 Balance 6 2 2 Balance 8.50.75 0.75 Balance

It will be evident based on the foregoing description that each of thecompositions in table 2 is primarily in the cubic phase due to the factthat the total dopant concentrations are in the range of 10–12 mol. %.More durable, erosion resistant coatings can be produced by lowering thetotal dopant concentration below about 6 mol. %, preferably about 4 to 6mol. %, to produce a solid solution incorporating substantial regions inthe tetragonal t′ phase as explained above. Desirable examples include1.33 mol. % primary stabilizer, 1.33 mol. % group A dopant and 1.33 mol.% group B dopant, balance zirconia, as well as 1.67 mol. % primarystabilizer, 1.67 mol. % group A dopant and 1.67 mol. % group B dopant,balance zirconia. Particularly desirable ceramic alloy solid solutionsbased on these compositions for the tetragonal t′ phase, and on thecompositions in table 2 for the cubic phase, have been made where yttria(Y₂O₃) is the primary stabilizer, ytterbia (Yb₂O₃) is the group Adopant, and gadolinia (Gd₂O₃) is the group B dopant. The 1.33-1.33-1.33composition just described has been found to be highly suitable forproducing very durable ceramic thermal barrier coatings for turbineblades in jet engines. It is believed that suitable tetragonal t′ phasecompositions also can be prepared based on the proportions of thecompositions listed in table 2, by multiplying each componentconcentration by about ⅓. For example, the analogous tetragonal t′composition for the cubic 4-3-3 composition in table 2 would be1.33-1-1. In this case, the sum of all three dopant concentrations is3.33 mol. %, which is below the ˜4 mol. % threshold for a substantiallytetragonal t′ phase coating. Alternatively, the multiplier can be adifferent value, such as in the range of ⅓ to ½ (e.g. 0.4), so long asthe resulting sum of all three dopant concentrations does not exceed the˜4 mol. % threshold. A similar calculation could be performed for eachof the cubic phase compositions in table 2 to achieve an analogoustetragonal t′ phase composition. In a further alternative, where a mixedphase having substantial regions of cubic and tetragonal t′ phases isdesired, other suitable multipliers can be used to provide an analogouscomposition having the same component proportions as any of thecompositions listed in table 2, where total dopant concentration is inthe range of about 4–6 mol. %.

The 4-3-3 composition where the individual components are selected to bethe species as in the preceding paragraph also is highly suitable forproducing a cubic phase coating for those turbine blades where lowintrinsic (and accordingly superficial or effective) thermalconductivity is a more important consideration than hardness orresistance to thermal cycling. In addition, the 4-3-3 and 6-2-2compositions discussed above also are believed highly suitable forcoating jet engine combustor components.

In general, when zirconia is selected as the base oxide, the compositionof the ceramic alloy solid solution preferably isZrO₂—Y₂O₃—REa₂O₃—REb₂O₃ (base oxide—primary stabilizer—group Adopant—group B dopant) where REa is a first rare earth oxide whosecationic radius is smaller than y³⁺ in the solid solution, and REb is asecond rare earth oxide whose cationic radius is larger than y³⁺ in thesolid solution. In this embodiment, most preferably REa is scandia(Sc₂O₃) or ytterbia (Yb₂O₃), and REb is neodymia (Nd₂O₃), gadolinia(Gd₂O₃) or samaria (Sm₂O₃), and all the components are present in amolar percent concentration according to one of the compositions listedin tables 1 or 2 above or in one of Examples 1–4 below.

When hafnia is used as the base oxide, it has been found that a dramaticdecrease in intrinsic thermal conductivity similar to that observed forzirconia-based systems also is found in hafnia-based cubic phasesystems, but in the range of 15–20 mol. % total dopant concentration.Accordingly, hafnia-based ceramic alloy solid solutions havingcompositions of 11-2-2 and 16-2-2 have been found to be desirable.Formulations according to these component concentrations also have beenmade using yttria, ytterbia and gadolinia respectively for the primarystabilizer, group A and group B dopants, and have been found to exhibitdesirably low intrinsic thermal conductivity and enhanced sinteringresistance.

In addition to the group A and B dopants mentioned above, it has beenfound that the addition of a third additional dopant in relatively smallamounts can have a substantially beneficial effect on further reducingthe intrinsic thermal conductivity of the resulting ceramic alloy solidsolution, as well as reducing the tendency for the coatings according tothe invention to crack due to thermal shock. Preferably, the thirdadditional dopant is scandia (Sc₂O₃) and is in addition to the group Aand group B dopants already being added to the composition. (In otherwords, though scandia is listed hereinabove as a preferred group Adopant, in this embodiment when it is presented as a third additionaldopant or “overdopant,” it is not intended as the group A dopant, and adifferent group A dopant oxide, preferably ytterbia is used as the groupA dopant oxide). The resulting scandia-modified, low conductivitycluster oxide thermal barrier coating provides an optimal combination oflow thermal conductivity performance and coating durability. When asmall amount, preferably in the range of 0.5 to 6 mol. %, morepreferably 0.5–1 mol. %, of Sc₂O₃ is added to the previously disclosedfour component, low conductivity thermal barrier coatings(ZrO₂—Y₂O₃—REa₂O₃_REb₂O₃, where REa and REb are rare earths other thanY), one can effectively reduce and/or tailor the thermal expansioncoefficient of the resulting oxide systems. The addition of scandia tothe clustered oxide systems also enhances the oxide atomic bondingbecause of the high affinity between scandium and oxygen, thus resultingin high chemical stability and intrinsic strength of the modifiedoxides. Therefore, the thermal shock resistance and coating long-termdurability of the modified thermal barrier coatings can be greatlyimproved while maintaining the very low coating thermal conductivity.The addition of this third additional dopant provides an overall5-component system, and the resulting effect on thermal conductivity isdescribed in the Examples below where the third additional dopant isreferred to as an “overdopant.”

The thicknesses of the ceramic thermal barrier coatings described hereinwill depend on the method of application, which in turn will be dictatedby the particular substrate to be coated and the service conditions itexperiences. Thicker coatings will be desirable for more strenuoustemperature and thermal cycling environments, whereas thinner coatingswould be suitable for less strenuous environments. The two principalmethods for applying these coatings are physical vapor deposition (PVD)and plasma spray. Both techniques are well known to those skilled in theart, and are not further described here except to note that PVDgenerally applies thinner coatings than plasma spray. For PVD appliedcoatings, e.g. onto turbine blades, coating thicknesses of 1–10,preferably 2–9, preferably 3–8, preferably 4–7, preferably about 5, milsare applied. For plasma spray applied coatings, e.g. onto a combustorsurface, coating thicknesses are somewhat higher, preferably 10–40,preferably 11–35, preferably 12–30, preferably 13–25, preferably 15–20,mils. Generally, thinner coatings are preferred for moving parts, andparticularly for parts moving at high speed in circular paths such asturbine blades, due to weight and centrifugal load considerations; theheavier the turbine blades (and their coatings) the greater thecentrifugal load on the turbine blade as it is accelerated in a circularpath, which can result in creep of the underlying metallic substrate.

For plasma sprayed coatings according to the invention, it is desirablethat the ceramic alloy powder used in the plasma spray process not havetoo many fines. Preferably, the ceramic alloy powder used for plasmaspray application of a ceramic thermal barrier coating according to theinvention will comprise not more than about 40, preferably about 35,preferably about 30, preferably about 25, preferably about 20,preferably about 15, preferably about 10, percent on a mass basis ofparticles smaller than about 325 mesh. It also is preferable that theceramic alloy powder be sprayed using plasma torch parameters which areselected so as to ensure that the particles are well-melted.

The following Examples (table 3) further illustrate various aspects ofthe invention. Examples 1–4 each show the components of a preferredembodiment of the invention, the amounts of the ingredients to producethe respective coating, and the resulting thermal conductivity observedafter 20-hour high temperature testing at about 1316° C. (2400° F.) insteady-state laser conductivity tests. The overdopant present in Example4 was selected to achieve a composition including two different speciesof group A dopants (i.e. dopants whose cationic radii are small thanthat of the primary stabilizer). Example 5 shows the baseline controlsample, a standard yttria-stabilized zirconia or 8YSZ. Percentages aregiven in molar percent.

TABLE 3 Example: 1 2 3 4 5 Base Oxide Zirconia Zirconia ZirconiaZirconia Zirconia balance balance balance balance balance Primary YttriaYttria Yttria Yttria Yttria Stabilizer 3 mol % 9 mol % 9 mol % 14 mol %4.5 mol % Group A dopant Ytterbia Ytterbia Ytterbia Ytterbia 0 1.5 mol %2.25 mol % 2.25 mol % 7 mol % Group B dopant Samaria Neodymia GadoliniaNeodymia 0 1.5 mol % 2.25 mol % 2.25 mol % 7 mol % Overdopant 0 0 0Scandia 0 0.75 mol % Thermal 0.72 0.63 0.70 0.56 1.35 Conductivity(W/m-K)

As these results demonstrate, the presence of the matched pairs of over-and under-sized cationic oxide dopants (group A and B dopants) accordingto the present invention reduces the 20-hour thermal conductivity of thecoatings. The coating sintering resistance at high temperature is alsosignificantly improved, as indicated by the significantly lowered ratesof the conductivity increase. The magnitude of increases in thermalconductivity after 20 hours in coatings according to the presentinvention were generally approximately 25–50% less than the magnitude ofincrease observed in similarly applied prior art 4.55 mol % yttriastabilized zirconia coatings. The foregoing results were surprising andunexpected.

Although the invention has been described with respect to preferredembodiments, it will be understood that various changes andmodifications can be made thereto without departing from the spirit andscope of the invention as set forth in the appended claims.

1. A thermal barrier coating composition comprising 46–97 molar percentbase oxide, 2–25 molar percent primary stabilizer, 0.5–25 molar percentgroup A dopant. and 0.5–25 molar percent group B dopant, said base oxidebeing selected from the group consisting of ZrO₂, HfO₂ and combinationsthereot said primary stabilizer being selected from the group consistingof Y₂O₃, Dy₂O₃, Er₂O₃ and combinations thereof, said group B dopantbeing selected from the group consisting of Nd₂O₃, Sm₃O₃, Gd₂O₃, Eu₂O₃and combinations thereof, and said group A dopant being selected fromthe group consisting of rare earth oxides, alkaline earth metal oxides,transition metal oxides and combinations thereof, but excluding thosespecies contained in said base oxide, group B dopant and primarystabilizer groups, wherein the ratio of the molar percentages of group Adopant to group B dopant in said composition is between about 1:10 endabout 10:1.
 2. A thermal barrier coating composition according to claim1, wherein the group A dopant is selected from the group consisting ofSc₂O₃, Tb₂O₃, MgO, NiO, Cr₂O₃, CoO, Fe₂O₃, TiO₂, RuO₂, Ta₂O₅, andcombinations thereof.
 3. A thermal barrier coating composition accordingto claim 1, said composition being a ceramic alloy solid solution havinga zirconia or hafnia lattice structure or structures, wherein the ionicradius of the group A dopant cation is smaller than the ionic radius ofthe primary stabilizer cation in said ceramic alloy solid solution.
 4. Athermal barrier coating composition according to claim 1, said group Adopant being Yb₂O₃, Sc₂O₃ or a mixture thereof.
 5. A thermal barriercoating composition according to claim 1, said group A dopant beingYb₂O₃, TiO₂, Ta₂O₅ or a mixture thereof.
 6. A thermal barrier coatingcomposition according to claim 1, said base oxide being ZrO₂, said groupA dopant being Yb₂O₃, said group B dopant being Gd₂O₃, and said primarystabilizer being Y₂O₃.
 7. A thermal barrier coating compositionaccording to claim 1, said group A and group B dopants each beingselected to have a high free energy of formation such that each of saidgroup A and group B dopants exhibits high stability in anoxygen-containing atmosphere.
 8. A thermal barrier coating compositionaccording to claim 1, said group A and group B dopants being selected toprovide a mixture of dopants effective to promote complex defectstructures and improved lattice scattering in said thermal barriercoating composition.
 9. A thermal barrier coating composition accordingto claim 1, comprising 0.5–12.5 molar percent group A dopant, and0.5–12.5 molar percent group B dopant.
 10. A thermal barrier coatingcomposition according to claim 9, wherein the group A dopant is selectedfrom the group consisting of Sc₂O₃, Yb₂O₃, MgO, NiO, Cr₂O₃, CoO, Fe₂O₃,TiO₂, RuO₂, Ta₂O₅, and combinations thereof.
 11. A thermal barriercoating composition according to claim 9, said composition being aceramic alloy solid solution having a zirconia or hafnia latticestructure or structures, wherein the ionic radius of the group A dopantcation is smaller than the ionic radius of the primary stabilizer cationin said ceramic alloy solid solution.
 12. A thermal barrier coatingcomposition comprising a ceramic alloy solid solution baying a baseoxide lattice structure or structures, said solid solution comprising46–97 molar percent base oxide, 2–25 molar percent primary stabilizer,0.5–25 molar percent group A dop ant, and 0.5–25 molar percent group Bdopant, said base oxide being selected from the group consisting ofZrO₂, HfO₂ and combinations thereof, said primary stabilizer beingselected from the group consisting of Y₂O₃, Dy₂O₃, Er₂O₃ andcombinations thereof, each of said group A dopant and said group Bdopant being selected from the group consisting of rare earth oxides,alkaline earth metal oxides, transition metal oxides and combinationsthereof, but excluding those species contained in said base oxide andprimary stabilizer groups, wherein said group A dopant is selected suchthat the ionic radius of the group A dopant cation is smaller than theionic radius of the primary stabilizer cation in said solid solution,and wherein said group B dopant is selected such tat the ionic radius ofthe group B dopant cation is larger than the ionic radius of the primarystabilizer cation in said solid solution, the ratio of the molarpercentages of group A dopant to group B dopant in said solid solutionbeing between about 1:10 and about 10:1.
 13. A thermal barrier coatingcomposition according to claim 12, said solid solution comprising 4molar percent primary stabilizer, 3 molar percent group A dopant,3 molarpercent group B dopant, balance zirconia and impurities.
 14. A thermalbanier coating composition according to claim 12, said solid solutioncomprising 4 molar percent primary stabilizer, 4 molar percent group Adopant, 4 molar percent group B dop ant, balance zirconia andimpurities.
 15. A thermal barrier coating composition according to claim12, said solid solution comprising 6 molar percent primary stabilizer, 3molar percent group A dopant, 3 molar percent group B dopant, balancezirconia and impurities.
 16. A thermal barrier coating compositionaccording to claim 12, said solid solution comprising 6 molar percentprimary stabilizer, 2 molar percent group A dopant, 2 molar percentgroup B dopant, balance zirconia and impurities.
 17. A thermal barriercoating composition according to claim 12, said solid solutioncomprising 8.5 molar percent primary stabilizer, 0.75 molar percentgroup A dopant, 0.75 molar percent group B dopant, balance zirconia andimpurities.
 18. A thermal barrier coating composition according to claim12, said base oxide being zirconia, said solid solution comprising asubstantially cubic crystalline phase having a total dopantconcentration, including primary stabilizer, group A and group Bdopants, of about 9–13 molar percent.
 19. A thermal barrier coatingcomposition according to claim 18, said total dopant concentration beingabout 10–12 molar percent.
 20. A thermal barrier coating compositionaccording to claim 12, said solid solution comprising 1.33 molar percentprimary stabilizer, 1.33 molar percent group A dopant, 1.33 molarpercent group B dopant, balance zirconia and impurities.
 21. A thermalbarrier coating composition according to claim 12, said solid solutioncomprising 1.67 molar percent primary stabilizer, 1.67 molar percentgroup A dopant, 1.67 molar percent group B dopant, balance zirconia andimpurities.
 22. A thermal barrier coating composition according to claim12, said base oxide being zirconia, said solid solution comprising asubstantially tetragonal t′ crystalline phase having a total dopantconcentration, including primary stabilizer, group A and group Bdopants, of about or less than 4 molar percent.
 23. A thermal barriercoating composition according to claim 12, said base oxide beingzirconia, said solid solution being a mixed phase composition comprisingsubstantial regions of tetragonal t′ and cubic crystalline phases, saidsolid solution having a total dopant concentration, including primarystabilizer, group A and group B dopants, in the range of about 4–6 molarpercent.
 24. A thermal barrier coating composition according to claim12, said base oxide being zirconia, said primary stabilizer beingyttria, said group A dopant being ytterbia and said group B dopant beinggadolinia.
 25. A thermal barrier coating composition according to claim12, said base oxide being zirconia, said primary stabilizer beingyttria, said group A dopant being scandia or ytterbia and said group Bdopant being neodymia, gadolinia or samaria.
 26. A thermal barriercoating composition according to claim 12, said base oxide being hafnia,said solid solution comprising a substantially tetragonal t′ crystallinephase having a total dopant concentration, including primary stabilizer,group A and group B dopants, of about or less than 10 molar percent. 27.A thermal barrier coating composition according to claim 12, said baseoxide being hafnia, said solid solution comprising a substantially cubitcrystalline phase having a total dopant concentration, including primarystabilizer, group A and group B dopants, of a boat or greater than 15molar percent.
 28. A thermal barrier coating composition according toclaim 12, said base oxide being hafnia, said solid solution being amixed phase composition comprising substantial regions of tetragonal t′and cubic crystalline phases, said solid solution having a total dopantconcentration, including primary stabilizer, group A and group Bdopants, in the range of about 10–15 molar percent.
 29. A thermalbarrier coating composition according to claim 12, said solid solutioncomprising 11 molar percent primary stabilizer, 2 molar percent groupdopant, 2 molar percent group B dopant, balance hafnia and impurities.30. A thermal barrier coating composition according to claim 12, saidsolid solution comprising 16 molar percent primary stabilizer, 2 molarpercent group A dopant, 2 molar percent group B dopant, balance hafniaand impurities.
 31. A thermal baiter coating composition according toclaim 12, said solid solution further comprising, in addition to saidgroup A dopant, 0.5 to 6 molar percent scandia as an additionaloverdopant.